Lithium rechargeable batteries (i.e. secondary lithium batteries, in which lithium ions are the principal charge carriers) are important devices in the field of energy storage. They offer advantages over other secondary battery technologies due to their higher gravimetric and volumetric capacities as well as higher specific energy.
Secondary lithium batteries fall into two classes—those in which the negative electrode is lithium metal, known as a “lithium metal battery”, and those in which the negative electrode is comprised of a lithium intercalation material, known as “lithium-ion batteries”. In terms of specific energy and power, lithium metal is the preferred negative electrode material. However, when ‘traditional’ solvents are used in combination with lithium metal negative electrodes, there is a tendency for the lithium metal electrode to develop a dendritic surface [1]. The dendritic deposits limit cycle life and present a safety hazard due to their ability to short circuit the cell—potentially resulting in fire and explosion. These shortcomings have necessitated the use of lithium intercalation materials as negative electrodes (creating the well-known lithium-ion technology), at the cost of additional mass and volume for the battery.
Researchers have continued to search for a solution to the poor cycling characteristics of the lithium metal electrode—notably through the use of polymer electrolytes. However lithium ion motion in polymer electrolytes is mediated by segmental motions of the polymer chain leading to relatively low conductivity. The low conductivity and low transport number of the polymer electrolytes has restricted their application in practical devices. Alternative solvents such as 1,3 dioxolane have been trialled with some success (i.e. uniform lithium deposition morphology) but have been found to react with lithium during cycling of the battery—thus the electrolyte eventually dries out and the battery fails prematurely [2].
In another field, since their first observation in 1927, various parties have studied room temperature ionic liquids (RTILs) and their potential applications. Room temperature ionic liquids are organic ionic salts having a melting point below the boiling point of water (100° C.). Accordingly, within this class are organic ionic salts that are liquid over a wide temperature range, typically from below room temperature to above 200° C.
Room temperature ionic liquids have been known for a long time, although those studied before 1992 were moisture sensitive, which hampered the development of practical applications. In 1992 the first air and moisture stable ionic liquids were reported, and since then a large number of anion-cation combinations have been developed.
However, compared to other solvent systems, published research pertaining to the use of room temperature ionic liquids in lithium secondary batteries is sparse. Few, if any, of the systems proposed have been demonstrated to be capable of use in practice. Some systems reported contain air and moisture sensitive room temperature ionic liquids. Research on other systems indicates that the battery would have insufficient cycling efficiency or would be subject to severe limits on the possible charging/discharging rates. Other publications specify that the room temperature ionic liquids must be used in the solid phase. Moreover, little work if any has been reported to show whether the proposed systems enable lithium to be both taken up by the negative electrode, and importantly then released. Unless this is achievable, and demonstrated, it cannot be predicted the electrolyte will have utility in a secondary battery application.